Cube Beam Splitters for Different Values of Reflectance to Transmittance Coefficients of Light

C. WesoJowska*, E. Dobierzewska-Mozrzymas*, E. Idczak*

T. Marcinow*, J. Pietrasik*, B.Stolecki*

Cube Beam Splitters for Different Values of

Reflectance to Transmittance Coefficients of Light

Optical properties of films cemented in cube beam splitters were investigated in unpolarized and linearly polarized light. Optical coefficients were measured in the visible range of the spectrum for the angle of incidence on films a = 45°. Achro­ matic, unabsorbing beam splitters for the R : T ratio from 1:5 to 1:2 were obtained by means of a double dielectric film of the type L , where L, H are substances with a low and high refraction coefficient, respectively, and A0 is the wavelength for which the film thickness was controlled during evaporation. Unabsorbing beam splitters for the ratio R : T — 1:1 require

” / K \ l λ0\ I λ0\ / A0\

a five-film dielectric coating [7], Optical thicknesses of particular films are equal: A/^1.13 — j, L p .7 2 — j , H |1.13— j, L ^ 1.72 — j,

h |i.18-^°J. Besides optical properties of a simpler beam splitter for R : T = 1:1 with a mixed coating i.e. double dielectric film L l —°\, H l — \ and a thin inconel film, were presented.

\ 2 / \ 4 /

I. Introduction

Cube beam splitters with a metallic film, commonly used in optical measurements, are achromatic in the visible range, but have high absorption. A cube splitter with an inconel film splitting light with

R \ T 3:2, absorbs about 60% of light. The de­ pendence of the optical coefficients R, T on the wavelength λ of a cube beam splitter with an inconel film is presented in Fig. 1. The reflectance and

trans-Fig. 1. Reflection (R) and transmittance (T) coefficients versus wavelength λ for beam splitter with an inconel film (light

linearly polarized) R . T = 3:2

*) Instytut Fizyki Technicznej Politechniki Wroclawskiej, Wroclaw, Wybrzeze Wyspianskiego 27, Poland,

mittance coefficients are denoted by Rp, Tp for the parallel, and Rs, Ts for the perpendicular components of linearly polarized light. As is seen from Fig. 1, cube beam splitter with an inconel film is achromatic in the visible range of the spectrum. For many years investigations were carried out on cube beam splitters with the dielectric films instead of the metallic ones. The litterature available to us consists of papers [1-9]. Recently, Sokolova [9] published a paper dealing with prism beam splitters with dielectric films, obtained by a chemical method. The results of our investi­ gations on cube beam splitters with dielectric and mixed films evaporated in vacuum, are presented below.

II. Experimental procedure

Film beam splitters were evaporated in an Edwards Vacuum Coating Unit type 19E7 under the pressure

p = l x 10-5 Tr, on the rectangular prisms of BK 7

glass. During evaporation the base was not heated. After the evaporation the prisms with films were glued together with the prisms without films to make a cube. For CaF2, MgF2, N a3AlF6 and ZnS a molyb- denium boat, and for inconel a tungsten filament were used as heaters. During the preparation of the

films the following parameters were fixed in all processes: substrate preparation, ionic bombardment cleaning, substrate rotation while cleaning and pre­ ssure during the evaporation. Thickness was checked photometrically by measuring maximum or minimum of the reflected light for a given wavelength λ0, selected by an interference filter. Two-, three-, and five-film coatings were obtained in one evaporation process.

The optical coefficients R and T as functions of the wavelength λ were measured for the angle of incidence a = 45°. The transmittance coefficients for unpolarized light were measured by a type VSU-1 Zeiss spectrophotometer The reflectance R and transmittance T coefficients for linearly polarized light were measured by means of a polarization spectro­ photometer. Radiation was detected in a photo­ multiplier directly coupled with recorder pen GiBi (Zeiss made). Using the same spectrophotometer, after removing the Nicol, the optical coefficients R and T for unpolarized light were determined. The transmittance and reflection coefficients were measured with the accuracy A T = 0.5% and AR — 2%, re­ spectively. Besides the ratio of white light reflected

(R) to transmitted (T) through the splitter, was

measured.

(fi — angle of refraction in the film. It seems to us

that a better achromatism of the splitter can be assured by a set of two dielectric films. The first film evaporated on glass has a low refraction coe­ fficient and optical thickness I— , whereas the second

\ 2 /

one has a high refraction coefficient, and optical thickness • To control film thickness during the evaporation of such beam splitters in an Edwards

r

R [V

•CO «50 100 150 600 550 T00 DO 000

Fig. 3. R = /(A), T = /(A) for beam splitter with a double film N a3AlF6 i ^ l , ZnS ( — I, A0 = 510 nm, R : T = 1:2. Beam splitter without antireflecting coatings. Light unpolarized

111. Experimental results and discussion

Generally, in the case of small value of the ratio

R . T a beam splitter with one dielectric film is pro­

posed. Optical thickness of the film is equal n^dv 1

cos®! = — λ0, where n t is the refraption coefficient 4

of a substance with a high refraction coefficient, e.g. T i0 2, dl is the geometrical thickness of the film,

1 R

f&J

— T

too 650 500 530 600 650 700 750 BOO A .nm

vacuum apparatus, the interference filter for λ0= 5 lOnm was used. By means of double dielectric films of the

1A0

type L H 1 ^ we obtained achromatic unabsor­ bing beam splitters for the rations R . T from 1:5 to 1:2. Here, L, H denote the substances with low and high refraction coefficients, respectively. In Figs 2 and 3 optical properties of beam splitters with a double dielectric film for unpolarized light are presented. In Fig. 2 the dependence of the reflectance R and transmittance T coefficients on the wavelenght

λ for a beam splitter with a double film CaF2

T

JO \ Γ

\

R

3 we

Fig. 2. R = /(A), T = /(A) for beam splitter with a double diele­ ctric film CaFj | — J , ZnS j , A0 = 510 nm, R : T = 1:5. Beam­

splitter without antireflecting coatings. Light unpolarized

| and Z n s |- ^ - j is given. In Fig. have R = f(X),T = f(?.) for a beam splitter with the films Na-,A1F6 Z n S j ^ j . In Fig. 4 optical properties of the same beam splitter in the linearly polarized light are presented. Me­ asurements in the polarized light were restri­ cted to the range 450-700 nm, because of the photomultiplier sensitivity range. Other beam splitters with double dielectric films investigated by us exhibited similar optical properties in polarized and unpolarized light. Beam split­ ters of this type are rather easy to obtain,

Fig. 4. Reflectance ( R) and transmittance (Γ) coefficients as functions of Λ in a linearly polarized light. Beam splitters with a double film N a3AlF6

| j,

(

j,

light. Beam splitter without aatireflecting coatings

as we control the thickness of evaporated films for integer multiplicity of . Using different su­ bstances for films with low and high refraction coefficients we can get different ratios R : T from 1:5 to 1:2. It is more difficult to get unabsorbing cube beam splitters with the ratio 1:1.

type. The following substances were chosen: zinc sulphide (films 1 and 3), cryolit (films 2 and 4), cerium oxide (film 5). Optical thickness of the respective

K

films was: n,d, - n3d3 = 1.13 — , n2d2 = nAd^ = 4

1.72— , n5ds = 1.18 — . During the evaporation thic-

4 2

kness of the film was controlled by an interference filter for λ0 = 553 nm. In Figs 5 and 6 the optical properties of a five-film cube beam splitter are pre­ sented. As can be seen in the Fig. 5 such a beam splitter is achromatic in the range 450-750 nm. Optical

Fig. 6. Reflection (Rp, Rs) and transmittance (Tp , T, ) coeffi­ cients versus wavelength A for a beam splitter with five-film dielectric coating. R : T = 1:1. Beam splitter without antire­

flecting coatings

properties of a beam splitter of this type in the linearly polarized light are presented in Fig. 6. The beam splitter investigated by us shows similar optical properties to those reported by Anders. The weak points of this beam splitter are the low values of the coefficients Rp and Ts. The beam splitter with mixed dielectric and metallic films may be regarded as a substitutional solution. Pohlack [6] has proposed a ZnS film of optical thickness and a thin

chro-Fig. 5. R — /(λ), T = /(A) for a beam splitter with a five-film dielectric coating. Λ : Γ = 1 : 1 . Beam splitter without antire­

flecting coatings. Light unpolarized

The optical properties of five-film beam splitters were presented by Anders [7]. Optical thickness of particular films should fulfil the following conditions:

λ0 λ0 λ0

«jt^coso?! = ■■— ,n2d2cosw2 = — , n 3d3cosw3 = — >

' 4 4 4

n4d4cosQ>4. = — , nsdscos<p5= — . Sokolova [9] pro-

4 2

poses a ten-film prism beam splitter, i.e. two rectan­ gular prisms with five-film dielectric coating obtained by a chemical method, glued up a cube beam splitter. We dealt with a five-film beam splitter of the Anders

Fig. 7. R = /(A), T = /(A) for a beam splitter with films N a3AlF6 | ^ | , Z n s |- ^ - j and inconel. Λ : Γ = 1 : 1 . Light

r

The authors would like to thank the Polish Optical Works in Warsaw for permission to publish this paper.

« « 500 550 m 650 700 750 600 λ,ΠίΏ

Fig. 8. Reflection (Rp , Rs) and transmittance (Tp , T s) coeffi­ cients versus wavelength A for a beam splitter with dielectric-

/A0\ I λ0\

-metallic films N a3AlF6 | - J, ZnS | - J and inconel. Light linearly polarized. Beam splitter without antireflecting coatings

mium film. We employed a double dielectric film of the type L and a thin inconel film. Optical properties of the beam splitter of this type are presented in Figs 7 and 8 in unpolarized and polarized light, respectively. As seen from Figs 7 and 8 this beam splitter is achromatic for the visible light, and has a lower absorption than a beam splitter with the inconel film, only.

Cube diviseurs optiques pour de differentes valeurs de coefficients de reflexion de transmission de lumiere

On a etudie des proprietes optiques des couches minces, colies en cubes diviseurs optiques, pour la lumiere polarisee et nonpolarisee. On a mesure des coefficients optiques pour Tangle d ’incidence sur la couche a = 45°. A l’aide du

revete-oii L, H sont des substances a bas et haut indice de refraction, et A0 est longueur d’onde de controle de l’epaisseur de la couche sous vide, on a re?u des cubes diviseurs optiques acromatiques, nonabsorbants pour differentes valeurs R :T comprises entre 1:5 a 1:2. If faut un revetement de cinq couches dielectriques

/

aux epaisseurs optiques des couches particulieres: H \ 1,

l ( ‘ ' 7 2 M ’ Η ( ( , · , 3 τ ) · L ( ’ ’ 72 4°) H ( U 8 y ) pour obtenir des cubes diviseurs optiques nonabsorbants pour des valeurs/?:T = 1 : 1 . On a presente ainsi Ies proprietes optiques d'un simple cube diviseur pour R : T 1:1 avec un revetement mixte une double couche dielectrique L Ao \— I et une

4 I couche mince d'inconel.

IV. Conclusions CBeToaeJiamne κνδιικιι ajih pa3JiHHHbix 3HaHeHuu κο3φφΗΚΗεητοΒ oTpawenmi n nponycKanna ceeTa

On the basis of the above presented investigations with cube beam splitters with dielectric and dielec­ tric-metallic films, the following conclusions can be drawn:

a) by means of double dielectric films of the type unabsorbing and achromatic cube beam splitters can be obtained. Using different substances with low and high refraction coefficients, different ratios R . T from 1:5 to 1:2 in the visible range can be obtained. Beam splitters of this type are obtained by means of a simple technology,

b) the cube beam splitters with five-film dielectric coating are achromatic, and unabsorbing for R : T = 1:1 in the visible range of the spectrum. The weak point of such beam splitters is their strong polarization of light, which may produce uneven illumination of the field of vision in certain optical nstruments, despite the achromatism of the beam splitter. The beam splitter with dielectric-metallic films may serve as a substitutional solution.

fljiH ecTecTBeHHoro, a T ax*e jiMHewHo — noJiapwiOBaHHoro ceeTa HCCjreaoBaHbi οπτκΗ εαοιε CBOiicTBa CBeToae/iamHx iuie-

hok, 3aKJieeHHbix b CBeToaejiamue Ky6wcn. M3MepeHbi οπτη- necKHe K03(|)(|)HuneHTbi b βηλημοη ofiaacTH cnexTpa a n a y m a naaeHHfl Ha mieHKy a = 45° C noM ouibio λβομηογο

ΛΗ3ϋεκ-TpHsecKoro ποκρωτΗ» Tuna L j, H (L, H o6o3Ha4a-k)t eemecTBa c hhckhmhbucokhm κοτφφηυΗεΗτοΜ npejioMJie- HHH, A0 HBJiaeTCH TOH ΛΛΗΗΟΗ BOJIHbl, πρΗ ΚΟΤΟΡΟΗ BO BpeMH Β03ΓΟΗΚΗ npOH3BO/IHJICH KOHTpOJlb TOJIUJHHbl) nOJiyHeHbl 6e3- a6copnuHOHHbie, axpoMaTHiecKHe CBeToaejiumwe Ky6m<H nun cooTHouieHHii R : T = 1 : 5, 1 : 3 , 1 : 2 . Ee3a6copuHOHHbie CBeToaejiHTenH λιιη εοοτΗοιηεΗΗΗ R : T = 1 : 1 Tpe6yK)T 5-cjioiiHoro 5iH3JieKTpH4ecKoro ποκρωτκΗ ο οπτΗτεεκοίί

τοη-uiHHe OTflejibHbix πηεΗοκ H

H ΠρΗΒΟΛΒΤΟΗ

TaK-xce onmnecKHe CBOHCTBa npocToro CBeToaeiiHTejiJi Bjia cootho- ηιεΗΗΗR : T — 1:1 cocMemaHHbiM ποκρΜτκεΜ — λβοηηθη anonc-KTpHHeKca» nneHKa Tana L

(τ)·Η(τ) *

References

[1] Hass G ., Vacuum, 2, 331 (1962).

[2] Holland L., Putner T., Bateman S., JOSA vol. 47, No. 7, 668 (1957).

[3] HollandL., Vacuum Deposition of Thin Films, Chapman a. Hall Ltd. London (1956).

[4] Catalan L. A., Revue d’Optique, Tome 42, No. 2, 53 (1963).

[5] Mathur K. C., Indian Journal of Pure and Applied Physics, vol. 4, No. 10, 394 (1966).

[6] PohlackH., Feingeratetechnik 7, No. 4, 171 (1958). [7] Anders H., Dunne Schichten fur die Optik Wissenschaftliche

Verlagsgesellschaft MBH Stuttgart (1965).

[8] Macleod H. A., Thin-Film Optical Filters, Adam Hilger LTD, London (1969).

[9] Sokolova S. R., Optico-Mechan. Promysl. t. 37, No. 5, 50 (1970).